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TMP01FP Scheda tecnica(PDF) 11 Page - Analog Devices |
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TMP01FP Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 20 page TMP01 Rev. E | Page 11 of 20 SWITCHING LOADS WITH THE OPEN-COLLECTOR OUTPUTS In many temperature sensing and control applications, some type of switching is required. Whether it be to turn on a heater when the temperature goes below a minimum value or to turn off a motor that is overheating, the open-collector outputs OVER and UNDER can be used. For the majority of applications, the switches used need to handle large currents on the order of 1 A and above. Because the TMP01 is accurately measuring temperature, the open-collector outputs should handle less than 20 mA of current to minimize self-heating. The OVER and UNDER outputs should not drive the equip- ment directly. Instead, an external switching device is required to handle the large currents. Some examples of these are relays, power MOSFETs, thyristors, IGBTs, and Darlingtons. Figure 17 through Figure 21 show a variety of circuits where the TMP01 controls a switch. The main consideration in these circuits, such as the relay in Figure 17, is the current required to activate the switch. TEMPERATURE SENSOR AND VOLTAGE REFERENCE VREF VPTAT 1 2 3 4 8 7 6 5 HYSTERESIS GENERATOR WINDOW COMPARATOR TMP01 R1 R2 R3 MOTOR SHUTDOWN 2604-12-311 COTO IN4001 OR EQUIV. 12V Figure 17. Reed Relay Drive It is important to check the particular relay to ensure that the current needed to activate the coil does not exceed the TMP01’s recommended output current of 20 mA. This is easily deter- mined by dividing the relay coil voltage by the specified coil resistance. Keep in mind that the inductance of the relay creates large voltage spikes that can damage the TMP01 output unless protected by a commutation diode across the coil, as shown. The relay shown has a contact rating of 10 W maximum. If a relay capable of handling more power is desired, the larger contacts probably require a commensurately larger coil, with lower coil resistance and thus higher trigger current. As the contact power handling capability increases, so does the current needed for the coil. In some cases, an external driving transistor should be used to remove the current load on the TMP01. Power FETs are popular for handling a variety of high current dc loads. Figure 18 shows the TMP01 driving a p-channel MOSFET transistor for a simple heater circuit. When the out- put transistor turns on, the gate of the MOSFET is pulled down to approximately 0.6 V, turning it on. For most MOSFETs, a gate-to-source voltage, or Vgs, on the order of −2 V to −5 V is sufficient to turn the device on. Figure 19 shows a similar circuit for turning on an n-channel MOSFET, except that now the gate to source voltage is positive. For this reason, an external transistor must be used as an inverter so that the MOSFET turns on when the UNDER output pulls down. TEMPERATURE SENSOR AND VOLTAGE REFERENCE VREF VPTAT 1 2 3 4 8 7 6 5 HYSTERESIS GENERATOR WINDOW COMPARATOR NC = NO CONNECT TMP01 R1 R2 R3 NC NC IRFR9024 OR EQUIV. HEATING ELEMENT 2.4k Ω (12V) 1.2k Ω (6V) 5% V+ + Figure 18. Driving a P-Channel MOSFET TEMPERATURE SENSOR AND VOLTAGE REFERENCE VREF VPTAT 1 2 3 4 8 7 6 5 HYSTERESIS GENERATOR WINDOW COMPARATOR NC = NO CONNECT TMP01 R1 R2 R3 NC NC IRF130 2N1711 4.7k Ω V+ 4.7k Ω HEATING ELEMENT Figure 19. Driving an N-Channel MOSFET Isolated gate bipolar transistors (IGBT) combine many of the benefits of power MOSFETs with bipolar transistors, and are used for a variety of high power applications. Because IGBTs have a gate similar to MOSFETs, turning on and off the devices is relatively simple as shown in Figure 20. The turn-on voltage for the IGBT shown (IRGBC40S) is between 3.0 V and 5.5 V. This part has a continuous collector current rating of 50 A and a maximum collector-to-emitter voltage of 600 V, enabling it to work in very demanding applications. TEMPERATURE SENSOR AND VOLTAGE REFERENCE VREF VPTAT 1 2 3 4 8 7 6 5 HYSTERESIS GENERATOR WINDOW COMPARATOR NC = NO CONNECT TMP01 R1 R2 R3 NC NC IRGBC40S 2N1711 4.7k Ω V+ 4.7k Ω MOTOR CONTROL Figure 20. Driving an IGBT |
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